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  • November 21, 2024, 08:12:28 AM

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Author Topic: Heater bypass valve vacuum source?  (Read 2154 times)

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Offline knobbler

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Heater bypass valve vacuum source?
« on: July 15, 2024, 12:15:59 AM »
Hey all! Got a couple questions if anyone can lend a hand:

Assume a completely empty engine bay, except for the single vacuum line that connects to the heater core bypass valve, and the valve itself. I see the line goes through the firewall, but I'm stumped as to where it goes from there. That leaves me with two questions:

1) What does this vacuum line connect to in the car? I believe it should be the heater control panel (specifically the bottom temperature adjuster), as from what I understand, setting it to "COLD" should cause the valve to switch to bypass the heater core.

2) What's the path and ultimate source of vacuum for this valve?

I verified the valve switches as it should when I manually apply direct vacuum, then connected the valve to the line (attached with the temp control set to "WARM"), but there's no corresponding movement in the valve switch when I set it to "COLD ." The engine and all other vacuum related stuff is out of the engine bay at the moment, so I'm hoping to figure out what this vacuum circuit should be before I start putting things back together. I assumed there was a mechanically actuated diaphragm in the dash that acted as the vacuum source, but I'm now guessing this may not be the case and would like to confirm.

If anyone can point me to a diagram or the relevant section in the TSM to help sort this stuff out, I'd appreciate it!
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Online Trooper

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Re: Heater bypass valve vacuum source?
« Reply #1 on: July 15, 2024, 04:57:20 AM »
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Offline knobbler

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Re: Heater bypass valve vacuum source?
« Reply #2 on: July 15, 2024, 06:54:03 AM »
Quote from: Trooper on July 15, 2024, 04:57:20 AM
This may help...

https://amceaglesden.com/guide/Diagram_Section

It did, thanks! I'm not sure why I didn't see the pair of lines and the switch they attach to when I was looking at the control panel diagram in my copy of the TSM, but I managed to spot them in the scanned image at that link. That gives me a good start to back trace things and get this set up correctly. Thanks again!
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Offline MIPS

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Re: Heater bypass valve vacuum source?
« Reply #3 on: July 15, 2024, 04:27:50 PM »
If I recall from pulling the HVAC out of another wagon. The AC heater core bypass valve gets its vacuum source from manifold vacuum where the SelectDrive and Vacuum gauge get it. There's a coffee can booster in between and possibly a check valve so the valve does not open from weak vacuum under hard acceleration.

I've considered adding the valve to a non-AC eagle because in the summer even with the blend door switch to Cool the heater core still operates and *any* air leaks in the foam seals can make an already hot day even worse.
« Last Edit: July 15, 2024, 04:30:15 PM by MIPS »
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Offline knobbler

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Re: Heater bypass valve vacuum source?
« Reply #4 on: July 16, 2024, 01:22:45 AM »
Quote from: MIPS on July 15, 2024, 04:27:50 PM
If I recall from pulling the HVAC out of another wagon. The AC heater core bypass valve gets its vacuum source from manifold vacuum where the SelectDrive and Vacuum gauge get it. There's a coffee can booster in between and possibly a check valve so the valve does not open from weak vacuum under hard acceleration.

I've considered adding the valve to a non-AC eagle because in the summer even with the blend door switch to Cool the heater core still operates and *any* air leaks in the foam seals can make an already hot day even worse.

Yeah, that's pretty much what I'm seeing so far as well. Haven't been able to dig in that much, though, since I was getting the plenum box in and the headliner resurfaced today.

I have an AC wagon, but I cut out the compressor and related underhood stuff because it was all pretty much dead and it's gonna be a while before I have the bandwidth to get a functioning system in place. Still keeping that valve, though, for exactly the reasons you mention — especially the foam. I don't have the time to totally refurb that plenum box, and 30 year old foam just doesn't seal like it used to!
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Offline MIPS

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Re: Heater bypass valve vacuum source?
« Reply #5 on: July 16, 2024, 10:17:15 PM »
AC eagles have a two-piece plenum box, if I recall. Much easier to pull the main blend door assembly out without disturbing the evaporator.
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Offline knobbler

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Re: Heater bypass valve vacuum source?
« Reply #6 on: July 16, 2024, 10:55:59 PM »
Quote from: MIPS on July 16, 2024, 10:17:15 PM
AC eagles have a two-piece plenum box, if I recall. Much easier to pull the main blend door assembly out without disturbing the evaporator.
They do, thankfully, so it'll be easier when the time comes for me to refurb the hvac stuff. For now I'm focused on engine/cooling/trans (with a slight deviation to the headliner, because the foam powder was killing my sinuses), then it's suspension, and after that I can focus on comfort stuff haha
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